The present invention relates to a composite material, in particular for reflectors, having a metallic support in strip form, an intermediate layer and having a multilayer optical system which is applied to the intermediate layer.
To characterize the reflective properties of a composite material of this nature, the spectral coefficient of the total reflection and that of the diffuse reflection are measured (DIN 5036). Different demands are imposed on the optical properties of reflectors. In many cases, the aim is to have a composite material with a high total reflectivity, i.e. with low reflection losses. If, in addition, a mirror nature, i.e. a low scattering of the light at the reflector surface or a low level of diffuse reflection, is required, the production of such a composite material involves a very high level of outlay in production engineering terms. In some cases, a higher level of diffuse reflection, and in certain circumstances even an absorptive behaviour with regard to the total reflectivity, is also desired. In some special applications, the electromagnetic radiation wavelength which is to be reflected may lie in the UV or in the IR range.
The base material used for reflectors with a high total reflectivity and a low diffuse component, is generally rolled aluminium of a minimum purity of 99.8%, for which, since the crude aluminium has a sensitive surface, an intermediate layer has to be applied in order to protect against mechanical and chemical influences and thus maintain its suitability for use. Such a protective intermediate layer is formed in a wet-chemical process which is referred to overall as anodizing and comprises an electrobrightening treatment and an anodic oxidation. By changing the purity and/or the roughness depth, it is possible to influence the level of total reflection, while the level of diffuse reflection can be influenced by making controlled changes to the rolled structure. In the wavelength range of visible light (380 to 780 nm), the total reflection of composite materials of this nature is from 83% to 87%. Considerable process engineering outlay, primarily a high outlay on rolling technology, as well as a high level of purity of the material used, are required in order to achieve a low level of diffuse reflection, in particular of below 4%, resulting in high costs. Despite these drawbacks, this type of semi-finished product in strip form has become established throughout the world as a standard product for the lighting industry.
Furthermore, it is known, when using less high-grade aluminium support material in strip form, to apply to the support a layer of eloxal which consists of Al2O3 in a thickness of approx. 1 to 3 xcexcm and is then applied to a multilayer optical system. The reduced thickness eloxal layer allows the surface to be sufficiently rough and hard and to be free from defects. A highly reflective high-purity aluminium layer is deposited on this eloxal layer. This high-purity aluminium layer is optically dense, about 50 to 80 nm thick and has a total reflection of more than 91% in the visible light range. Yet more layers which increase the reflection can be applied to this reflective layer. For example, a subsequent continuous strip coating which is described in the journal xe2x80x9cMetalloberflxc3xa4chexe2x80x9d 50 (1996), 10, improves the optical efficiency of such aluminium strips up to a total light reflectivity of approx. 95%. However, the diffuse light reflectivity of material with a mirror nature is improved only to an insignificant extent. The optically active surface has a good mechanical load-bearing capacity and sufficient chemical resistance for use as a reflector material. However, the intermediate eloxal layer which serves as a smoothing layer is relatively brittle, so that there is a risk of cracks being formed in the event of extreme mechanical deformation processes.
A significant drawback of the two known optically active composite materials described above is that they are limited to aluminium as the support material. This entails complex, sometimes environmentally disadvantageous process steps during production, such as wet-chemical brightening, anodizing and, if appropriate, the vacuum coating or the need to use high-purity, specially rolled and therefore expensive starting material.
U.S. Pat. No. 5,527,562 has disclosed, as the intermediate layer, a levelling application to an aluminium support which serves as a substrate for a subsequent vacuum coating with an optically reflective layer system. This application is of an organic silicon sol which is applied to suitably pretreated aluminium. The special sol is intended to bring about good levelling of the aluminium-strip surface and high total reflectivities after the coating in vacuo by sputtering of Al, SiO2, TiO2. A drawback of this arrangement is that the use of a very specific formulation for the sol layer necessitates a high process engineering outlay.
Furthermore, it is known that curved, rigid reflector bodies (e.g. for headlights) are provided with a levelling coating layer, to which a single-layer reflective application of aluminium is then applied in vacuo, and this in turn is provided with a protective layer. A drawback of this is that the total reflectivity does not exceed that of the aluminium layer, i.e. approx. 90%, and that the resultant surface does not exhibit sufficient mechanical strength. In particular, the surface is so unable to withstand loads imposed by forming operations that under standard industrial conditions further processing would be impossible.
The present invention is based on the object of providing a composite material, in particular for reflectors, of the type described in the introduction, with which it is possible, in a less complex and therefore less expensive manner, as required, to achieve any desired total reflectivity, in particular even values of over 95%, and any desired level of diffuse reflection, in particular even values of below 4% (according to DIN 5036) and which has a high mechanical strength and chemical resistance. The composite material should not be limited to a support made from aluminium and should be highly deformable.
According to the invention, this is achieved by the fact that a non-metallic protective layer, which consists of a low-absorption material and has a thickness of from 5 to 20 nm, preferably from 5 to 10 nm, is applied to the multilayer optical system.
The protective layer may preferably be a sputtered layer, in particular a layer produced by reactive sputtering, a CVD or PECVD layer or a layer which is produced by vaporization, in particular by electron bombardment or from thermal sources. The protective layer may consist predominantly of a material with a low refractive index, such as for example SiO2.
The invention opens up entirely new technical and economic possibilities relating to the production of composite reflector materials. In particular, depending on requirements, it is possible, without reducing the mechanical strength of the composite material, to apply an eloxal layer or, in particular, a coating layer with a thickness which is adapted to the roughness of the support, which may characteristically lie within the range from about 2 to 20 xcexcm, for the intermediate layer. The coating material may be a polycondensate produced on the basis of one or more monomers, a polyadduct or a polymer produced in particular by free-radical polymerization. Irrespective of the basis on which the coating-material mixture is produced, it is possible, by suitably selecting the mixture and/or the application process and/or the curing parameters, to set slight or, as far as possible, complete levelling of the support surface, resulting in a planar surface with a minimal roughness depth or a desired undulating or rough structure. Thus, with regard to its influence on the reflectivity, the coating material has the same importance as the rolled surfaces of the known aluminium strips.
In this case, the protective layer plays the major role. According to the current state of scientific knowledge in the field of optical physics, each further layer which is applied to a multilayer optical system comprising, for example, a metallic reflective layer and two interference layers positioned above it has a considerable adverse effect on the optical characteristics of the layer system. Surprisingly, according to the invention, it has proven possiblexe2x80x94in contrast to this generally recognized scientific knowledgexe2x80x94to significantly increase not only the mechanical load-bearing capacity values (DIN 58165, part 5), but also to increase the reflectivity by approx. 1% point.
A further important aspect of the invention is the possibility of using an inexpensive metallic support which may consist of simple industrially rolled aluminium. However, it is also possible, according to the invention, to use other metallic support materials, such as magnesium, copper, titanium, molybdenum, tantalum or steel, such as for example stainless steel, or alloys containing these substances, such as for example brass, thus making it possible, on the one hand, to improve the mechanical properties of the composite material so as to achieve higher strength values, but, on the other hand, also allowing materials which are less expensive than aluminium to be used. The surface roughness of these materials is no longer important, since the surface structure of the composite material according to the invention is determined primarily by the intermediate layer, and the thickness of the intermediate layer can be adapted to the roughness of the support material.
Advantageously, the entire process of producing the composite material according to the invention can take place in a continuous process: the multilayer optical system and the protective layer can be applied to the intermediate layer using the continuous vacuum strip-coating process, while for the intermediate layer it is possible in particular to use formulations which can be applied using the coil-coating process, dried and, if appropriate, structured. These formulations are in particular stoving enamels with a toughness which can be preset by means of organic or inorganic solvents and which can be cured at temperatures of up to about 250xc2x0 C. and are based on acrylic resins, epoxy resins, phenolic resins, melamin resins, urea resins or polyurethane resins. The curing takes place predominantly as a result of a crosslinking reaction due to activation of the double bonds which are present in the monomers. The enamel layers formed are distinguished by a high scratch resistance, extensibility and insensitivity to corrosive influences.
The coating materials used for the structure forming layer should predominantly be those which exhibit such flexural adhesion to the support that there is no delamination of the intermediate layer when a support with a material thickness of 0.4 mm is bent around a mandrel with a diameter of 2 mm and an adhesive strip of type Scotch 670 CFM, produced by 3 M, which has been previously stuck on and is also bent around the mandrel is pulled off.
Furthermore, it is advantageous, with regard to a vacuum coating which takes place after the application of the intermediate layer, to use a coating material which has a glass transition temperature of over 100xc2x0 C. and a gas release rate of less than 1*10xe2x88x924 mbar 1 Sxe2x88x921 mxe2x88x922.